In the field of electric power tools, higher capacity and reduced weight is demanded for batteries driving cordless tools. In order to meet these demands, high power density lithium ion batteries are adopted in some cases. In lithium ion batteries, charging is primarily conducted by constant current and constant voltage control. One method of detecting full charge involves utilizing the property of the charging current decreasing once the charging voltage of the lithium ion battery reaches the maximum voltage. For example, Patent Literature 1 proposes a method wherein the charging current is detected during charging, and the battery is determined to be fully charged when the detected charging current falls below a predetermined current value.    Patent Literature 1: Unexamined Japanese Patent Application KOKAI Publication No. H2-192670